Abstract The distance patients need to travel to care facilities is a recognized barrier to accessing heath care. However, little is known about the role of travel burden on patient choice of treatment strategy, adherence to that treatment strategy, and, ultimately, treatment outcomes. Women diagnosed with early stage breast cancer have two guideline-supported treatment options: 1) mastectomy and 2) breast-conserving surgery (BCS) followed by radiation therapy (RT)1-3. While overall survival between the two choices is equivalent, BCS+RT is associated with reduced complications and costs4 and superior quality of life relative to mastectomy5. Distance, however, may play a significant role in patient decision-making, since RT requires daily travel to radiation facilities for several weeks at a time. The combination of two clinically comparable treatment options with different travel implications provides a unique opportunity to evaluate the impact of travel distance on care decisions and outcomes. I will use the linked Surveillance, Epidemiology and End Results (SEER)-Medicare 2004-2014 data, which covers 28% of the total U.S. population and contains detailed information on patient demographics and place of residence, tumor characteristics and disease severity, location of treatment received, and patient survival. Using the linked claims data will allow me to assess distance traveled, minimum distance required, whether a patient actually received RT following BCS20, whether the treatment was completed, and over how long a period. The 10-year time period allows me to evaluate changes in the availability of radiation facilities. Under Aim 1, I will quantify the burden of travel and how it has changed over time among breast cancer patients in the Medicare population and identify patient demographic factors (age, race/ethnicity, marital status, income, rurality) associated with greater travel burden. Under Aim 2, I will evaluate the association between travel distance and 1) patient choice of surgical treatment (mastectomy or BCS) and 2) adherence to a complete course of RT among BCS patients. Under Aim 3, I will exploit the variation in the availability of radiation facilities over time caused by facility opening or closure to determine how patients in a given geographic area respond to changes in access to radiation facilities. I will also estimate the patient survival hazard ratio associated with living in a county with versus without a radiation facility. This research will contribute to the larger discussion surrounding the importance of efficient resource allocation, incentivizing practice in underserved locations, the impact of consolidating health care markets, and improving access to needed services for rural and remote populations.